Dark Matter

Most of the matter in the universe is neither bound up in stars or planets nor dispersed in clouds of "ordinary" particles. But what is it made of?

Map of dark matter (light blue), digitally superposed on a photograph made by the Hubble Space Telescope, shows that a giant ring of invisible mass surrounds the dense core of a giant cluster of galaxies called ZwC10024+1652, about 5 billion light-years from Earth. Astronomers mapped the distribution of mass in the galaxy cluster by observing the effects of gravitational lensing on background galaxies.

NASA, ESA, M.J.Jee and H. Ford (Johns Hopkins University)

First and foremost, dark matter—matter that emits neither light nor any other detectable form of radiation—is real, notwithstanding the struggles of a small minority of physicists to explain it away. It was created immediately after the big bang, 14 billion years ago, and has persisted ever since, forming the bulk of all the matter in the cosmos. In spite of its mysteries, dark matter is detectable through a web of observations that complement and support one another. In fact, American and European physicists are racing to catch its invisible particles in new, ever improving detectors. What excites them is the sense that they are closing in on the answer to one of the great cosmic riddles: What is most of the universe made of?

What makes astronomers so sure that dark matter exists? The answer is gravity. All matter, including invisible matter, exerts gravitational forces on the matter we can see.

Fritz Zwicky, the prickly Bulgarian-Swiss-American astronomer who was the first to conclude that dark matter must exist, introduced the concept in 1933. By applying Newton’s laws and measuring the speeds of individual galaxies within a cluster of galaxies, Zwicky could deduce the mass of the cluster. He also determined the amount of visible matter in the clusters by measuring the brightness of the galaxies that form them. Those two measurements showed that a typical giant cluster of galaxies comprises at least ten times more invisible matter than what is visible. Later observations would rule out the possibility that the invisible matter is all made up of diffuse gas floating among the galaxies. Such intergalactic gas does exist, but in nothing remotely like the quantities needed to account for most of the dark matter.

Zwicky’s conclusions gained scant attention from his colleagues. The snub was partly provoked by his cantankerous nature—he referred to fellow astronomers as “spherical bastards,” meaning that they were bastards no matter how you looked at them. But a greater hurdle was the revolutionary implication of his idea: few could accept that most of the universe remained to be discovered.

So dark matter suffered three decades of neglect. Then in the 1970s two astronomers at the Carnegie Institution of Washington (D.C.), Vera S. Rubin and W. Kent Ford Jr., mapped the motions of stars within galaxies close to our own Milky Way. They reached essentially the same conclusion as Zwicky had: each galaxy includes enormous amounts of dark matter, far more than all the luminous stuff in the galaxy’s stars. The bulk of it forms a giant, dark halo extending far beyond the star-strewn galactic expanses that we see.

Astronomers today, applying Zwicky’s logic, are still detecting vast quantities of dark matter in distant galaxy clusters. Among the clusters, they have observed clouds of hot gas, which would have escaped the clusters’ gravitational pull billions of years ago if the clusters had no more mass than that of their stars.

Impressive as those observations are, there’s even more evidence for the unseen presence of dark matter: the phenomenon of “gravitational lensing.” Because gravity bends space itself (Einstein’s finest insight into nature), light passing close by a massive object deviates from a straight-line trajectory. Hence if a massive object happens to lie almost directly along our line of sight to a more distant source of light, such as a galaxy, the light we see will be bent or even focused, much as if the intermediate object were an optical lens [see illustration below]. A small amount of light bending, or “lensing,” can distort the galaxy into an unusual shape, just as the thick glass bottom of an old Coke bottle distorts the shape of a light bulb when you look at the bulb through the bottle. Stronger lensing can actually create multiple images of the same light source. Gravitational lensing enables astronomers to map the distribution of all matter, not just visible matter, because all matter can give rise to a lensing effect.

Gravitational lensing can occur when light from a distant galaxy, center left, passes through a dark-matter halo around a cluster of galaxies. Here the gravitational pull of the dark matter deflects the light in such a way that an observer on Earth sees two additional images of the galaxy. The diagram is highly idealized; the distances and angles are not drawn to scale. Illustration by Ian Worpole (www.ianworpole.com)

What, then, is this dark matter that makes up by far the bulk of all the matter in the universe? No one knows. But cosmologists do know one thing for sure: most of it cannot be anything like the matter familiar to us.

Cosmologists classify all matter into two kinds: baryonic and nonbaryonic, or, basically, the ordinary and the exotic. “Baryon” comes from the Greek root barys, meaning “heavy”; the term was coined to refer to the heavy particles that fuse together in the nuclei of ordinary atoms—neutrons and protons. They far outweigh the electrons, which are leptons, or “light” particles, not baryons. With the realization that matter exists in more exotic forms, the term “nonbaryonic” came to denote not only leptons but also all other particles that do not participate in nuclear fusion. One of the most important clues to the mystery of dark matter comes from the growing evidence that the bulk of it—and thus, most of the matter in the universe—is nonbaryonic matter.

Baryonic matter forms stars, planets, moons, and even the interstellar gas and dust from which new stars are born. Nonbaryonic matter includes neutrinos, tiny particles each having less than a millionth the mass of the already diminutive electron. Neutrinos were once regarded as likely candidates for dark matter because they exist in such prodigious numbers, but they have now been excluded from the dark-matter sweepstakes. Detailed studies of how galaxies form suggest that dark matter is most likely made of particles whose masses range from roughly that of the proton to several hundred times as much.

How do astrophysicists infer that such hypothetical particles of dark matter must be nonbaryonic? They can estimate the total amount of matter from the effects of gravitational lensing and the distribution of cosmic background radiation. The baryonic part of that total then comes from the current understanding of how the cosmos behaved during its earliest epochs. The big bang, with which the universe began, opened an era of nuclear-fusing fury, a time when all particles crowded together at unimaginably high densities and temperatures. All creation then resembled the cauldron at the core of a star, only far more so. From the countless nuclear fusions that took place in those first few minutes after the big bang, there emerged the basic ratio of nuclei in the universe today: almost entirely hydrogen and helium, with only a minute smattering of all heavier nuclear varieties.

By the end of its first few minutes, the universe had expanded and cooled, dipping below the billion-degree temperatures needed for nuclear fusion. Only in much later, highly localized events did the stars cook up almost all the heavier elements, such as the carbon, nitrogen, oxygen, silicon, and iron that make up our planet and ourselves. Those heavier nuclei, however, comprise no more than 2 percent of the mass of all baryonic matter. The other 98 percent is still made up of hydrogen, helium, and their isotopes, created immediately after the big bang. By measuring the relative amounts of the various isotopes of hydrogen and helium nuclei, cosmologists can deduce how much baryonic matter took part in the great crucible of cosmic nuclear fusion in the first half hour of the universe.

Those results, now confirmed by detailed studies of the cosmic background radiation, lead to a startling conclusion. Baryonic matter—some of it in stars, but much more in diffuse interstellar gas-—forms no more than a sixth of all matter in the universe. The other five-sixths must be nonbaryonic matter, either in the form of elementary particles or clumped into much larger objects.